Solar Attic Fan (Replace Wired One)

I currently have wired power vent in my Attic, it Quit working. Not sure
what size it is, but I wanted to replace it with Solar version. I am not sure how
to calculate what size fan to replace it with or if one fan is even enough,

The 20 watt solar fan I found so far moves 1300 CFM, How do I calculate
what size fan I need,

My attic does have ridge vents as well and get feed from the sofits

Is it hard to calculate the volume of the attic? What do you use to determine
the volume of air you need to exchange to keep attic cooler.

I read on the web if you have ridge you really do not need power fan

I can tell hugh difference in the temp of the attic w.o the power fan. I so not even know the size fan the builder used. I am sure its the lowest CFM

Divide the attic into two right triangles (i.e.,measure from the peak to the rafters, then from the point right under the peak at the rafters to the outside edge of the attic), find the area of each, and then multiply by the length. So, if it was 8' from the rafters to the peak and 10' to the outside edge, it would be 8x10x.5 or 40 sq ft. Then say it was 20' long, take 40x20 or 800 cubic feet. Do this for each side. You want probably at least a few air turnovers an hour. See what the manufacturers recommend for air changes. My guess is 5-8, but that's a guess.

If you have eave vents, you'll get some natural convection...aiding it can help with a fan.

I think you might have some decimals crossed. 1300 CFM is a LOT of air, and not consistent with a 20 watt load on a fan. A typical bath fan, which might draw something less than 100 watts, moves 50 to 100 CFM.